The present invention relates to a seat belt retractor having multi-level load limiting.
A seat belt retractor generally comprises a cylindrical bobbin or spool with a circular cross-section. Seat belt webbing is attached to and wound around the spool and the spool is mounted on a spool shaft to be rotatable in the retractor to wind in webbing under action of a retractor spring and to pay-out webbing under the influence of relatively gentle forward directed movement of a vehicle occupant, for example to allow for normal movement associated with vehicle occupancy such as reaching forwards to activate in-car controls (for a radio or a window) or to reach a glove compartment or door pocket. In the event of a crash situation, the more extreme momentum of the vehicle occupant activates a crash sensor which locks the spool against rotation and thus prevents forward motion of the vehicle occupant and injury due to impacting the interior structure of the vehicle such as the steering wheel, dashboard or windscreen.
However, this sudden locking of the seat belt spool in a crash can sometimes cause injury to the vehicle occupant due to sudden impact of the torso with the belt webbing. This is particularly true in severe crashes. In recent years this problem has been recognized and some solutions have been proposed.
One known proposal has been described in U.S. Pat. No. 5,014,927, wherein load limiting is effected by interposing a plastically or elastically deformable member in the force path, for example between the spool and the innermost winding of the belt webbing. Particularly common currently is the use of a torsion bar, made of steel that twists when high torque is applied and can rotate up to seven or eight times before breaking. The pay-out of webbing is generally in proportion to the momentum of the vehicle occupant at the moment a crash is sensed. This pay-out of webbing lessens the otherwise severe restraining forces on the vehicle occupant at high vehicle speeds, especially during the initial moments of the crash.
However in known seat belt systems using torsion bars the force absorbed is constant, i.e. the force at low speeds, with small, light vehicle occupants, is the same as at high speeds with large vehicle occupants, and the force constant over the whole period of load limiting. It would be preferable to tailor the load limiting to absorb a larger load at the beginning of the crash when forces are higher and to reduce the load limiting level as the vehicle occupant contacts an airbag. This reduction in load limiting level would reduce chest injuries to the vehicle occupant and reduce chest deflection that is measured during crash simulations.
There is provided in accordance with one aspect of the present invention a seat belt retractor comprising: a spool rotatably mounted in a frame for retraction or pay-out of seat belt webbing depending upon the rotation direction of the spool; a locking ring attached to one end of the spool; a means for locking the locking ring against rotation when a crash is sensed; and load limiting means for allowing further pay-out of the seat belt webbing after the spool has locked, under the influence of a vehicle occupant""s forward momentum, wherein the load limiting means comprises a torsion bar attached at one end to the locking ring and at the other end to the spool; and a resilient mechanism releasably coupled in the force path between the spool and the locking ring; and means for de-coupling the resilient mechanism after a predetermined number of turns of the torsion bar.
There is provided in accordance with another aspect of the invention a seat belt retractor comprising: a spool rotatably mounted in a frame; a locking ring attached to one end of the spool; a lockbar for locking the locking ring against rotation when a crash is sensed; and a multi-level load limiter for allowing further pay-out of seat belt webbing after the spool has locked, under the influence of a vehicle occupant""s forward momentum, wherein the multi-level load limiter comprises a torsion bar attached at one end to the locking ring and at the other end to the spool; and a wire releasably coupled in the force path between the spool and the locking ring such that during a first stage of load limiting both the wire and the torsion bar provide load limiting and during a second stage of load limiting the torsion bar alone provides load limiting.